THE PRODUCTS OF CATALYZED DIBORATION OF BIS(P-TOLYL)ETHYNE AND OF 4-CYANOPHENYL-ETHYNE BY BIS(CATECHOLATO-O,O')DIBORON

Both zodioxaborolan-2-yl)-1,2-bis(4-methylphenyl)ethene [(I), C28H22B2 O4] and is(1,3,2-benzodioxaborolan-2-yl)vinyl]benzonitrile [(II), C21H 13B2NO4] have a cis arrangement of two boronate ester substituents on a C=C double bond, which shows a slight twist to relieve steric crowdi ng of the substituents. The interplay of steric and electronic factors results in approximate coplanarity of only one substituent with the a lkene unit in (I), while this is achieved for two mutually trans subst ituents in the less crowded (II). A consistent pattern of bonding and conformational geometry is found for a series of related alkenes with boronate ester substituents. The catecholboronate group shows almost i nvariant geometry across a range of compounds, with a slight but proba bly real lengthening of the B-O bonds on coordination to transition me tals.